Gas turbine engines are known, and typically include a fan delivering air into a compressor section where the air is compressed. The compressed air moves into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, causing them to rotate, and in turn drive fan and compressor sections.
When the gas turbine engine is utilized for aircraft application, historically there have been at least two “spools,” including a first, low pressure spool and a second, high pressure spool. Typically, a high pressure turbine, downstream of the combustion chamber, is driven to rotate at a relatively high mechanical speed (hereafter “speed”) by very high pressure combustion gases. This high pressure turbine typically drives a high-pressure compressor via the high pressure spool.
Downstream of the high pressure turbine is a low pressure turbine. The low pressure turbine drives the low pressure spool, which typically extends within the inner periphery of the high pressure spool to reach a low pressure compressor. A fan may also rotate with the low pressure spool.
More recently the placement of a gear reduction configuration between the fan and the low pressure compressor has been proposed such that the two can rotate at different speeds. This allows the fan to rotate slowly enough to avoid the occurrence of complications related to high rotational speeds at the outer tips of the fan.
In addition, it has also been proposed to incorporate a gear reduction between the turbine and the low pressure compressor, such that those two can rotate at different speeds.
There are challenges with the above-referenced designs. First, there are a large number of bearings, and two shafts. Further, there are challenges with rotor dynamics, and in particular vibration issues, associated with a long, relatively thin low spool fit inside of a high spool. In addition, the amount of compression work provided by the high and low pressure compressor is somewhat constrained by the speed of high and low pressure turbines. The two turbines may require relatively large axial space. Also, various mount structures may be positioned between the turbine stages in the path of the hot combustion gases.